1. Filed of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a photocopier, a facsimile machine, a printer, etc., and more particularly to an image forming apparatus having an improved method of detecting the amount of toner adhered to a toner image on an image carrier in a precise manner for controlling a toner supply operation to obtain a desired image quality.
2. Discussion of the Background
An image forming apparatus using an electrophotographic image-forming process is known to perform at least the following process steps: forming an electrostatic latent image on an image carrier by an image-writing device; developing the electrostatic latent image with toner by a developing device; transferring the toner image onto a transfer member such as a transfer paper, directly or via an intermediate transfer member; and fixing the toner image on the transfer paper and outputting the transfer paper carrying the toner image as a final print.
In addition, when the image forming apparatus uses a two-component developer, various process controls, for example, a toner supplying control, a surface voltage control, and the like, are performed in order to obtain a desired image quality. It is known that the quantity (or density) of toner adhered to a toner image on an image carrier, such as a photoconductive member or an intermediate transfer member, is measured for controlling an operation of supplying toner to a developing device from a toner container to obtain a desired image quality.
FIG. 1 illustrates a typical example of a background method of measuring the quantity of toner on a photoconductive drum using a mirror-reflection sensor. The amount of toner on a toner patch 20 that is formed by toner on the photoconductive drum 200 is measured by a sensor 205. The sensor 205 includes a light-emitting device 10 and a light-receiving device 12. The light-emitting device 10 and the light-receiving device 12 are arranged such that infrared light, which is emitted by the light-emitting device 10 and which impinges upon the surface of the photoconductive drum 200 at a certain incident angle relative to the surface of the photoconductive drum 200, is reflected by the photoconductive drum 200 at the same angle as the incident angle, and is then received by the light-receiving device 12. The toner patch 20 has a shape, for example, as shown in FIG. 2. The intensity of the infrared light, which is reflected by the toner patch 20 and received by the light-receiving device 12, changes according to the quantity of toner on the toner patch 20, and thereby the output of the sensor 205 changes.
FIG. 3 illustrates an example of the output characteristics of the sensor 205 shown in FIG. 1. The intensity of light reflected by the toner patch 20 on the photoconductive drum 200 decreases as the amount of toner on the toner patch 20 increases up to a certain amount of toner, such that the output voltage of the sensor 205 decreases, and thereby the amount of toner on the toner patch 20 can be measured based upon the output voltage of the sensor 205. Specifically, as shown in FIG. 3, the output voltage for an amount of black toner on the photoconductive drum 200 decreases in inverse proportion to the amount of toner up to the amount "b", and is then saturated. Thus, the amount of the black toner can be measured only up to the amount "b". However, it may be desirable to measure the amount of the black toner up to the amount "c" which may be, for example, 1 mg/cm.sup.2. The reason why the output voltage for the black toner decreases as the amount of the black toner increases is that the reflection coefficient of the black toner is smaller than that of the photoconductive drum surface.
The output voltage for color toner on the photoconductive drum 200 has the characteristic that the output voltage decreases in inverse proportion to the amount of toner up to the amount "a", but less abruptly than the black toner, and then increases, as shown in FIG. 3. Thus, the amount of the color toner can be measured only up to the amount "a". However, it may be desirable to measure the amount of the color toner up to the amount "c". The reason why the output characteristic is changed is that the reflection coefficient of the color toner is smaller than that of the photoconductive drum surface, but is larger than that of the black toner when the amount of the color toner is small, and the reflection coefficient of the color toner becomes larger due to light scattering when the amount of the color toner is large.
FIG. 4 illustrates an example of another method of measuring the quantity of toner on a photoconductive drum with an irregular-reflection sensor. A sensor 205 includes a pair of a light-emitting device 10 and a light-receiving device 11. The light-emitting device 10 and the light-receiving device 11 are arranged such that the light-receiving device 11 receives light, which is emitted by the light-emitting device 10, incident on the surface of the photoconductive drum 200 at a certain incident angle and reflected by the photoconductive drum 200 at an angle differing from the incident angle. The amount of toner is measured in substantially the same manner as in the method using a mirror-reflection sensor.
FIG. 5 illustrates an example of the sensor output characteristics of the irregular-reflection sensor 205 shown in FIG. 4. Generally, color toner has a higher reflection coefficient than the photoconductive drum surface for an irregular-reflection light, and black toner has a lower reflection coefficient than the photoconductive drum surface. Consequently, when the amount of color toner on the photoconductive drum 200 increases, the output voltage of the sensor 205 increases, and when the amount of black toner on the photoconductive drum 200 increases, the output voltage of the sensor 205 decreases. That is, the irregular-reflection sensor 205 can measure the amount of both black and color toner up to desirable amounts of the black and color toner, such as, for example, 1 mg/cm.sup.2 respectively.
For precisely measuring the amount of toner on the photoconductive member surface by a sensor, it is desirable that the output characteristics of the sensor are always kept as shown in FIG. 5. However, various factors affect and change the output characteristics of the sensor 205 in actuality, which hampers precisely measuring the amount of toner on the photoconductive member.
For example, when scattered toner stains the sensor 205, the optical characteristics of the light-emitting device 10 and the light-receiving device 11 are affected, and the initial characteristics of the sensor 205 indicated by the solid line in FIG. 5 are changed, for example, to the characteristics indicated by the broken line in FIG. 6. The solid line in FIG. 6, which is the same as the solid line in FIG. 5, is reprinted for reference to the broken line.
Further, even when the sensor 205 is not stained by toner or deteriorated to cause a change in the optical characteristics, some parts which physically contact the photoconductive drum 200, such as a cleaning blade or a cleaning brush of a cleaning device, a transfer roller or a transfer belt, or an intermediate transfer belt of a transfer device, a developer-mix of a developing device, may damage a surface of the photoconductive drum 200. FIG. 7 illustrates an example of the sensor output characteristics of the irregular-reflection sensor 205 when the surface of the photoconductive drum 200 is damaged (indicated by the broken line) and when the surface of the photoconductive drum 200 is not damaged (indicated by the solid line). The damaged surface of the photoconductive drum 200 increases irregular-reflection. Therefore, the output characteristics of the sensor 205 are most effected when no toner is put on the surface, and as the amount of toner increases the influence of the damaged surface of the photoconductive drum 200 decreases, because the damaged surface of the photoconductive drum 200 is covered by the increased amount of toner.
Such a change in the output characteristics of the sensor 205 can be compensated, for example, by installing a standard reflecting member for reference and adjusting the output of the sensor 205 when the amount of toner on the photoconductive drum surface is measured by the sensor 205. However, installment of an additional component, such as the reflecting member, not only increases costs but also creates another problem that the additional reflecting member itself may be stained by toner.
Japanese Patent Publication No. 85184/1995 describes a sensor which has two light-receiving devices 11 and 12 as shown in FIG. 8, one for receiving a mirror-reflection light and the other for receiving an irregular-reflection light. The above JP No. 85184/1995 describes that the devices 11 and 12 are adjusted such that the difference of the two output signals of the devices 11 and 12 relative to a predetermined toner patch becomes a predetermined value, for accomplishing a stable measurement of the amount of toner adhered to a toner image on the photoconductive drum 200 even when the devices 11 and 12 are stained. The sensor 205, however, cannot be adjusted in response to a change in the reflection caused by damage on the surface of the photoconductive drum 200.
When the irregular-reflection sensor is used, the sensitivity of the sensor is typically adjusted to a predetermined level using a standard reflecting body prior to being installed in an apparatus, and is then adjusted again using an actual reflecting body, such as a photoconductive member, when the sensor is installed in the apparatus. However, when the surface of the photoconductive member has been damaged and the irregular-reflection sensor has such characteristics, for example, as indicated by the broken line in FIG. 7, if the sensor is adjusted by means of altering the intensity of light of the light-emitting device 10 so as to cause the output of the sensor relative to a part of the photoconductive member carrying no toner image to be the same as the one relative to a part of the photoconductive member not having been damaged, the output characteristics of the sensor may shift as indicated by the broken line in FIG. 9. As the diagram in FIG. 9 shows, the output characteristics of the sensor 205 shift vertically over the whole range, and thereby precise measurement of toner density can not be accomplished.